1. Field of the Invention
The present invention generally relates to a bumper collision sensor for an automobile and the like for detecting collision with a pedestrian.
2. Description of the Related Art
In recent years, from the viewpoint of protection of pedestrians in traffic accidents, there has been a worldwide tendency toward the establishment of pedestrian protection rules for automobiles. The rules aim at reducing a degree of injuries suffered by a pedestrian when an automobile collides with the pedestrian. Various pedestrian protection systems have been conventionally proposed. For example, there are a system for, when collision with a pedestrian is detected, lifting a hood to prevent a head of the pedestrian from colliding with a hard engine unit and a system for, when collision with a pedestrian is detected, expanding an airbag over a hood. These systems attract attention as systems for actively reducing head injuries of pedestrians and are developed extensively.
These pedestrian protection systems require a bumper collision sensor for detecting collision with a pedestrian. As such a bumper collision sensor, a bumper collision sensor using a load sensor is proposed. A plan view of the load sensor is shown in FIG. 14 and an output waveform chart of the load sensor is shown in FIG. 15.
In FIG. 14, load sensor 1 includes sensor cells 1b that are formed on belt-like sensor film 1a by screen printing or the like. When a load is applied to sensor cells 1b, a voltage, which is an output of sensor cells 1b, changes in proportion to a magnitude of the load. Plural sensor cells 1b are formed substantially at equal intervals in a longitudinal direction of sensor film 1a. Load sensor 1 with such a constitution is arranged in a longitudinal direction of a bumper of an automobile.
When the automobile collides with a pedestrian, a load corresponding to impact caused by the collision is applied to load sensor 1 arranged in the bumper. As shown in FIG. 15, a sensor output rapidly increases when the automobile collides with the pedestrian but then rapidly decreases when the pedestrian is sent flying. Therefore, a waveform of the sensor output has a peak when the automobile collides with the pedestrian. In this case, the peak is large when the automobile collides with an adult and is small when the automobile collides with a child.
On the other hand, when the automobile collides with a fixed object such as a wall or a pillar, a sensor output rapidly increases and then a load continues to be applied to the sensor cells 1b because the fixed object is never sent flying. As a result, as a waveform of the sensor output, the sensor output continues to increase gently without decreasing.
Consequently, load sensor 1, which is the conventional bumper collision sensor, can not only detect collision but also judge whether a collision object is a human or an object according to whether a peak value in time width T in FIG. 15 is in an output range for a human (between S1 to S2).
The conventional constitution is disclosed in JP-A-2004-276885.
However, although it is certainly possible to detect collision distinguishing a human and an object by using load sensor 1 in the bumper collision sensor, it is necessary to accurately obtain a waveform shown in FIG. 15 for the detection. Therefore, load sensor 1 has to be firmly fixed to the bumper.
On the other hand, from the viewpoint of pedestrian protection, in order to reduce damage to the legs of a colliding pedestrian to which damage are applied first, a shock absorbing structure, which is adapted to be easily deformed, tends to be adopted for a bumper. It is known that an amount of deformation of the bumper at the time when a pedestrian collides with the bumper is about a diameter of a femoral region of one leg. This is equivalent to about 15 to 20 cm in a standard physique.
Since load sensor 1 firmly fixed to the bumper is a belt-like sensor consisting of sensor films 1a, when load sensor 1 is subjected to such deformation due to collision, it is likely that load sensor 1 is broken during the deformation of the bumper if load sensor 1 is stretched on the order of several tens of centimeters. As a result, it is likely that a sensor output is interrupted during measurement of the waveform shown in FIG. 15 to make it impossible to detect collision.
To cope with the problem, it is conceivable to make sensor films 1a strong such that sensor films 1a are not broken. In this case, load sensor 1 is not deformed even if the bumper is deformed at the time of collision of a pedestrian. Thus, it is likely that load sensor 1 will hurt the legs of the pedestrian. When sensor film 1a is made of a soft material such as rubber, sensor cells 1b move according to deformation of the bumper. This could make it hard to measure an accurate load.